A hybrid vehicle refers to a vehicle equipped with a hybrid system, that is, a system using a combination of two types of power sources, such as a gasoline engine and an electric motor. Examples of the hybrid system include a series hybrid system (S-HEV), a parallel hybrid system (P-HEV), and a system (S&P-HEV) having the functions of both the series hybrid system and the parallel hybrid system. Hybrid systems are characterized in that they selectively employ either the gasoline engine or the electric motor depending on traveling conditions for a vehicle in order to obtain the advantages of the selected power source while compensating for the disadvantages of the unselected power source.
A series hybrid system uses a motor to drive wheels and has an engine operating as a power supply source for the motor. The series hybrid system is thus an electric vehicle with an engine generator. These systems are characterized in that they operate a low-power engine at an almost fixed rotation speed in an area where the engine can operate efficiently, to allow the vehicle to travel while performing efficient charging. On the other hand, the parallel hybrid system uses both the engine and the motor to directly drive the wheels. In this system, the motor supplies supplementary power to the engine while serving as a generator to charge a battery.
Thus, in both of the above-described systems, the hybrid vehicle uses a gasoline engine and an electric motor as power sources. Consequently, compared to conventional vehicles (ICE) of the same type using only the gasoline engine as a power source, the hybrid vehicle generally employs an engine with a smaller displacement (smaller engine swept volume).
On the other hand, the battery mounted in the hybrid vehicle as a power source has a limited life owing to degradation during use. That is, long distance traveling increases the internal resistance of the battery to gradually degrade the charging and discharging capability (battery capacity) of cells. Therefore, the battery in the hybrid vehicle must be replaced at appropriate intervals.
The degradation of the battery hinders the optimal balancing of the gasoline engine and the electric motor, which is a characteristic of the hybrid vehicle as described above. Consequently, the burden on the gasoline engine increase because the degradation of the battery over time impairs the capability of the electric motor which inherently serves as a power source, causing the automatic control to compensate by making greater use of the gasoline engine. As a result, the hybrid vehicle increasingly travels using the gasoline engine having a displacement smaller than that of the conventional vehicles (ICE) of the same type, resulting in a reduced gasoline mileage.
A significant decrease in gasoline mileage occurs more frequently in connection with driving patterns involving cornering, lane changes, acceleration, or deceleration, than in driving patterns in which the vehicle travels at a fixed speed, that is, during normal traveling. During normal traveling, the hybrid vehicle travels mainly using the gasoline engine, and is thus driven and operated in almost the same manner as that in which the conventional vehicle is driven and operated. On the other hand, in the driving patterns such as cornering, lane changes, and deceleration, the system uses both the gasoline engine and the electric motor to effectively utilize both power sources. These driving patterns are also characterized in that the gasoline mileage varies depending on, for example, the traveling speed or track of the vehicle or the driver's driving operation.
For the mileage (fuel efficiency) of conventional hybrid vehicle, for example, average gasoline mileage, instantaneous gasoline mileage, total gasoline mileage, and record-high gasoline mileage are measured and displayed. The display of these gasoline mileage values begins when an ignition switch is turned on. For example, the average gasoline mileage and the instantaneous gasoline mileage indicate the average gasoline mileage and instantaneous gasoline mileage measured every one minute (for about 30 minutes), respectively. The total gasoline mileage indicates the total gasoline mileage from the time of a data resetting operation until the present time. The record-high gasoline mileage indicates the greatest value between the times of the reset operations in the total gasoline mileage display.
As a conventional technique for dealing with battery degradation, a system has been disclosed which detects variation in charging and discharging capability among the cells in order to provide an early warning to urge the driver to perform uniform charging. That is, the charging and discharging capabilities of the cells in the battery are not uniformly degraded, but the degradation varies among the cells. If the variation is excessive, the life of the battery may be adversely affected. Thus, the driver is given a warning. In this case, all the cells can be refreshed to provide for a full charge condition by using an external charger to conduct a low current through the cells for a long time.
On the other hand, Japanese Patent Laid-Open No. 6-163084 discloses a method of measuring the remaining life of an electric vehicle battery. This measuring method determines the remaining life of the battery by measuring, each time the vehicle is operated, a voltage value obtained at a given discharge quantity during the travel, and, on the basis of the relationship between the voltage value and the corresponding number of journeys, determining the difference between the estimated number of journeys at which a preset lifetime voltage is reached and the present number of journeys.